Obstructive sleep apnea/hypopnea syndrome (OSAHS) is a well-recognized disorder that may affect as much as 1-5% of the adult population. OSAHS is one of the most common causes of excessive daytime somnolence. OSAHS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics.
OSAHS is associated with conditions in which there is anatomic or functional narrowing of the patient's upper airway, and is characterized by an intermittent obstruction of the upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea), despite continued respiratory effort, to significant obstruction with or without reduced airflow (hypopnea—episodes of elevated upper airway resistance, and snoring). Morbidity associated with the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the respiratory obstructions and arousals from sleep.
The pathophysiology of OSAHS is not fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment of the respiratory airway during the negative intraluminal pressure generated by inspiratory effort. The human upper airway during sleep behaves substantially similar to a Starling resistor which is defined by the property that flow is limited to a fixed value irrespective of the driving (inspiratory) pressure. Partial or complete airway collapse can then occur associated with the loss of airway tone which is characteristic of the onset of sleep and which may be exaggerated in OSAHS.
Starling resistor behavior is generally identified by the presence of an abnormal flow/pressure relationship. When the upper airway acts as a rigid tube (i.e., the normal state of the upper airway), flow is linearly related to a pressure difference across the upper airway. This relationship results in a substantially sinusoidal shape to a curve representing airflow in the airway over the time. When the upper airway exhibits Starling resistor behavior, the pressure/flow relationship changes. In particular, once the driving pressure decreases below a critical value, flow no longer increases in proportion to the pressure and a plateau develops on the pressure flow curve. This relationship produces a distinctive change in the shape of the inspiratory flow curve with respect to time. The detection of this abnormal shape of the inspiratory flow time curve, identifying an abnormal flow limitation, plays a critical role in both the diagnosis and treatment of OSAHS as it represents one of the least invasive means of detecting airway abnormalities.
Diagnosis of the spectrum of sleep disordered breathing requires the detection of all abnormal breathing events during sleep, including the occurrence of the abnormal inspiratory flow time contour indicative of Starling resistor behavior of the upper airway. While this may be performed by a human scorer, the automation of this analysis to provide rapid, reliable detection of such respiratory events is an important goal. Conventional apnea and hypopnea detection may be performed based on the analysis of signal amplitude alone. However, by definition, collapsible airway events showing Starling resistor behavior must be detected by other methods. Measurement of the pressure (drive) producing breathing is not practical in the majority of subjects requiring diagnosis.
Since 1981, positive airway pressure (PAP) applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for this disorder, and is now the standard of care. The availability of this non-invasive form of therapy has resulted in extensive publicity for sleep apnea/hypopnea and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of tracheostomy. Increasing the comfort of the system (e.g., by minimizing the applied nasal pressure) has been a major goal of research aimed at improving patient compliance with therapy.
In recent years, automatically adjusting PAP devices have been developed. These devices are designed to produce the appropriate PAP pressure needed to prevent obstructive respiratory events from occurring at each moment in time. The device may change the pressure (upward or downward) in response to characteristics of the patient's breathing. For example, the automatic PAP system must be able to accurately identify flow limitations and recognize them as indicative of a sleep disorder. Some systems have based flow limitation detection on empiric algorithms with user specified parameters that require adjustment for each patient to achieve optimal performance.